The present invention relates to apparatus and method for measuring an external parameter, eg, temperature, pressure or displacement.
It is sometimes required to measure external parameters (physical variables) in relatively inaccessible places where the use of electrical transducers for remote monitoring is inappropriate. For example, inside electrical machines such as transformers, generators or motors, no conventional electrical transducer can be used because of the high magnetic fields. It has been proposed to use optical techniques in such cases.
Some optical materials are known to exhibit a temperature dependence of some feature of their absorption spectrum. An optical thermometer can thus be made by arranging to pass light through such an optical material acting as a temperature transducer, and monitoring variations in the energy of light transmitted through the material with temperature. One such device is proposed in "An Optical Temperature Sensor for High Voltage Applications" by Saasky and Skaugset, in a paper given at the 7th IEEE/PES Transmission and Distribution Conference and Exposition, Apr. 1 to 6, 1979. This paper disclosed that selected materials appear to exhibit useful temperature dependent absorption spectra, but a preferred material is selenium ruby glass which exhibits a relatively sharp edge in its absorption spectrum where the absorption co-efficient rises from a relatively low value to a very high value rapidly with decreasing light wavelength. The location in the spectrum of this edge is temperature dependent so that the absorption co-efficient of such ruby glass to light at a specific wavelength on the absorption edge can be highly temperature dependent.
The above paper describes a possible temperature sensor comprising a pellet of the ruby glass sandwiched between a reflective coating and the end of a fibre optic bundle. Light from a suitable diode source is fed to the pellet along some of the fibres of the bundle passes through the pellet is reflected back through the pellet by the reflective coating and into the other fibres of the bundle to be fed to a photo-detector. The intensity of light detected should provide an indication of the temperature of the pellet.
This device has various drawbacks in that there may be other variables effecting the output for which the device does not compensate. Compensation is described in the paper only for any variations in the original light source.
A somewhat similar temperature sensor is described in "Fibre-optic Instrument for Temperature Measurement" by Kyuma, et al. in IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, April 1982. Again the temperature dependent absorption edge of a selected optical material is used as the sensing device. Two semi-conductor materials, CdTe and GaAs are disclosed. Two lengths of single optical fibre are used to convey light from a source through a body of the temperature sensitive material and then back again to the photo-detector. However, in this arrangement, the light source is arranged to emit two pulses of light at different wavelengths, one wavelength selected to be on the absorption edge of the sensing material, and the other wavelength selected to be substantially away from the absorption edge, in the transmission region of the material. The intensities of the light of the two wavelengths transmitted through the sensor material are then measured and compared. In this way it is said that the resulting device is made relatively insensitive to changes in the light detector output resulting from effects other than the temperature dependency of the absorption edge of the sensor material. For example, the device can have other temperature dependent losses in the light path from the light source to the detector including optical connector loss at the interfaces between the optical fibres and the body of sensor material, and the scattering and other loss co-efficient in the optical fibres themselves. If the two wavelengths of light used in this device are of the same order, then it can be expected that variations in the unwanted loss co-efficients in the light path will be the same at both wavelengths. Thus, comparison of the received signals at the two wavelengths enables the measured absorption co-efficient at the wavelength on the absorption edge of the sensor material to be normalised to minimise dependence on loss co-efficients in the system other than the desired absorption co-efficient of the sensor material.
If temperatures are to be measured at several locations with the above prior art temperature sensors, then separate optical fibre connections must be made from the monitoring point to the sensors at each location. Furthermore, the sensor described by Saasky and Skaugset is susceptible to loss co-efficients other than the desired absorption loss, whereas that described by Kyuma et al. is susceptible to variations in the emitted light output of the light source.
A fibre optic temperature distribution sensor has also been proposed by Hartog and Payne in a paper given to the Colloquium on Optical Fibre Sensors organised by Professional Group E13, Electronic Division, Institution of Electrical Engineers, on May 26th, 1982. In this paper, it is proposed to make use of the temperature dependence of the back scatter signal from an optical fibre using optical time-domain reflectometry. A particular example is described employing liquid core fibre with a liquid known to exhibit substantial variations in refractive index and Rayleigh scatter with temperature. In optical time-domain reflectometry, a pulse of light is launched into one end of the optical fibre and the energy back scattered from various parts of the fibre along its length is then monitored at the launching end. The technique has similarities to radar in that the distance along the fibre from the launching end from which the back scattered light originated is dependent on the time delay following launching of the light pulse. In the described example in the above paper, the back scattered energy received at the launching end at any particular time, corresponding to a particular distance along the fibre, is directly dependent on the temperature at that point along the fibre. However, the back scattered energy from a uniform fibre at a uniform temperature along its length, decays exponentially with time, and so analysing the back scattered energy signal to provide indications of the temperature along the length of the fibre can be difficult.
It will be appreciated that optical time-domain reflectometry is known for testing the properties of optical fibres, especially attenuation. This is a technique used to test fibres used to achieve desired low levels of attenuation for use of optical fibres in communication systems and the like. Conduit, Hartog and Payne have described an improved method of measuring the attenuation at various points along the length of the fibre, entitled "Spectral- and Length-Dependent Losses in Optical Fibres Investigated by a Two-Channel Back Scatter Technique", published in Electronics Letters, Vol. 16, No. 3, Jan. 31st, 1980. In this technique, the back scattered energy at two temporally spaced times is measured, the times corresponding to a pair of spaced points along the length of the fibre being tested. By comparing the energies from the two spaced points along the fibre, the attenuation between them can be determined and this value is normalised to minimise the effect of any variation in the energy of the light pulse launched into the fibre. The above article appears concerned solely with measuring the properties of an optical fibre for testing purposes.